During the process of hydrocarbon production under the natural geothermal pressure of a reservoir, the reservoir pressure will deplete. Eventually, the pressure in the reservoir may become too low to force fluid from the producing zone to the surface and artificial-lift may be required. In some cases, artificial-lift techniques are employed at the very onset of production, depending upon the overall techno-economic characteristics of the reservoir.
An artificial-lift system is defined as any system which adds energy to the fluid column in a wellbore, with the objective of initiating and improving production from the well. Artificial-lift methods fall into two groups, those that use gas and those that use pumps.
In a gas lift method, gas is injected into a well through valves placed along the wellbore at strategic points. The gas aerates fluid to make it less dense and consequently the reservoir pressure becomes sufficient to lift the oil and force it from the wellbore.
Methods that use pumps use a surface power source to drive a downhole pump assembly. The objective is to generate a large positive pressure gradient between the exit point of the pump and the surface, in order to increase the rate of fluid transport to the surface. In addition, downhole pumps serve to reduce pressure between the pump entry point and the wellbore interface with the reservoir. This increases the differential pressure between the reservoir and the wellbore, which in turn may increase the rate of fluid flow into the wellbore, providing the reservoir is compliant.
In its passage from a reservoir to the surface, fluid including hydrocarbons is arranged to flow through various orifices, openings or other constrictions. For example, when a pump is arranged in the wellbore, the fluid must pass through the inlet opening of the pump before it is lifted to the surface. Also, in some cases, wellbores may include associated sand control barriers (also known as “sand packs”, “sand screens” or “gravel packs”), upstream of a wellbore pump. The sand control barriers are provided for maintaining structural integrity of the wellbore in the absence of casing whilst still allowing fluid to pass from the reservoir into the wellbore and also control the migration of formation sand into the wellbore pumps and/or surface equipment. However, sand control barriers can act as a significant force working against the passage of fluid into the wellbore.
Disadvantageously, the passage of fluid including hydrocarbons through constrictions as described may produce a significant positive pressure which may reduce the rate of flow of fluid from the reservoir to the wellbore and/or into the wellbore pump.